1. Field of the Invention
The present invention relates to a power supply. More particularly, the present invention relates to the pulse width modulation (PWM) of a switching mode power converter.
2. Background of the Invention
The PWM is a traditional technology used in the switching mode power converter to control the output power and achieve the regulation. Various protection functions, such as over-voltage and over-current protection are built-in in the power supply to protect the power supply and the connected circuits from permanent damage. The function of output power limit is generally used for the over-load and short circuit protection. Referring to FIG. 1, a traditional PWM power supply circuit using the PWM controller 100, such as the PWM-control integrated circuit 3842, which has been widely used for the power supply, is illustrated. The operation of PWM-control starts on the charging of a capacitor 290 via a serial start-up resistor 222 when the power is turned on until the VCC reaches the threshold voltage, and then a PWM controller 100 starts to output a PWM signal and drive the entire power supply. After the start-up, the supply voltage VCC is provided from the auxiliary bias winding of the transformer 400 through a rectifier 330. The resistor 230 that is connected serially with the power MOSFET 300 determines the maximum output power of the power supply. The method is to connect the voltage of resistor 230 to the current-sense input (VS) of the PWM controller 100. If the voltage VS is greater than the maximum current-sense voltage such as 1V, the PWM controller 100 will disable the output of its OUT pin, and restrict the maximum power output of the power supply. The energy stored in an inductor is given by   ϵ  =                    1        2            xc3x97      L      xc3x97              I        2              =          P      xc3x97      T      
The maximum output power P can be expressed as follows:                               I          P                =                                            V              IN                                      L              P                                xc3x97                      t            ON                                              (        1        )                                P        =                                                            L                P                                            2                xc3x97                T                                      xc3x97                          I              P              2                                =                                                    V                IN                2                            xc3x97                              t                ON                2                                                    2              xc3x97                              L                P                            xc3x97              T                                                          (        2        )            
Ip and Lp are the primary current and the primary inductance of the transformer 400, respectively, tON is the turn-on time of the PWM signal in which the power MOSFET 300 is switched on, and T is the PWM switching period. From the equation (2), we found that the output power will vary as the input voltage varies. When the safety regulations are taken into consideration, the range of the input voltage is from 90Vac to 264Vac, wherein the output power limit of the power supply in high line voltage is many times higher than the output power limit in low line voltage. Although the output voltage (power) will be kept constant by automatically adjusting the tON through the feedback control loop of the power supply, the maximum tON is restricted when the the voltage in the VS pin is higher than an upper limit voltage, such as VSxe2x89xa71V(Ipxc3x97Rsxe2x89xa71V, where Rs is the resistor 230). Furthermore, the maximum output power is also affected by the PWM controller""s response time tD. From the moment that the voltage in the VS pin is higher than the upper limit voltage (Ipxc3x97Rsxe2x89xa71V) to the moment that the PWM controller 100""s OUT pin is actually turned off, there is a delay time tD. Within this delay time tD, the power MOSFET is still on, and it will continue delivering power. Therefore, the actual turn-on time of the PWM signal is equal to tON+tD, and the actual output power becomes as follows:                     P        =                                            V              IN              2                        xc3x97                                          (                                                      t                    ON                                    +                                      t                    D                                                  )                            2                                            2            xc3x97                          L              P                        xc3x97            T                                              (        3        )            
Although the tD time is short, generally within the range of 250xcx9c300 ns, the higher the operating frequency is, the more impact is caused by tD because the switching period T is short and tD becomes relatively more important. The input voltage VIN should be compensated properly, such that the input voltage will not affect the maximum output power. Referring to FIG. 1, a bias resistor 220 is added between VIN and the VS pin for compensation. The function of the bias resistor 220 can compensate the difference of the output power caused by the input voltage VIN and the delay time tD. By properly selecting the value of the bias resistor 220, an identical output power limit for the low line and high line voltage inputs can be obtained
The bias resistor 220 causes significant power consumption, especially in high line voltage input, it can be shown as follows:                               P          R                =                              V            IN            2                    R                                    (        4        )            
Besides, a high voltage across the resistor 220 causes inconvenience for the component selection and PCB layout.
The invention provides a PWM controller having a line voltage input that allows using one resistor for the functions of start-up resistor and bias resistor. The PWM controller comprises a current divider, a mirror-R, an adder and a reference voltage to start up the power supply and compensate the output power limit.
An input resistor is connected from the input voltage to the current divider to provide an input current for the PWM controller, wherein the variation of the input current is directly proportional to the change of the input voltage. The current divider includes two MOSFET""s. A first MOSFET transparently drives the input current to charge up the start-up capacitor. Once the voltage in the start-up capacitor reaches the threshold voltage, the PWM controller starts to operate. A second MOSFET proportionally mirrors a mirror current from the first MOSFET in accordance with the geometric size of the first MOSFET and the second MOSFET. The mirror current flows into the mirror-R to generate an offset voltage. Through the adder, the reference voltage subtracts the offset voltage and produces a programmable maximum current-sense voltage for the output power limit. Because the offset voltage is a function of the input voltage, the variation of the maximum current-sense voltage is inversely proportional to the deviation of the input voltage, and by selecting a proper input resistor an identical output power limit can be achieved for low line and high line voltage input.
In addition, the behavior of the mirror-R is a resistance, however it is difficult to design a precise resistor inside the integrated circuit. Thus producing a resistor with a precise absolute value in the integrated circuit is invented. The mirror-R comprises a constant voltage, a constant current, an operation amplifier (op amplifier), and two MOSFETs associated with two resistors to generate a precise mirror-R. The constant current flows into the drain of the first MOSFET, while the gate of the first MOSFET is driven by the op amplifier to make its drain voltage equal to the constant voltage. The second MOSFET is cross-coupled with the first MOSFET to mirror the resistance of the first MOSFET for the output. Two resistors are connected from the source of two MOSFETs to the ground respectively to expand the linear resistance region of the MOSFETs.
Advantageously, the PWM controller having a line voltage input for output power limit of the present invention can provide functions for starting up the power supply and compensating the output power limit. Furthermore, only one resistor is applied, which saves the power consumption, eases the PCB layout, and shrinks the size of power supply.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.